Paralleling multiple power converters is a common practice in the telecom and UPS (uninterruptible power supply) industries to increase overall system power capacities and to enhance system reliabilities by building redundancy. Typical examples of such power converters are single phase or three phase converters comprising inverters, rectifiers and DC/DC converters. Typically all the parallel power converters are gated synchronously and are tied together through isolation transformers to limit the cross current. Synchronous gating implies that the gate controls for the parallel converters are perfectly aligned.
Another way to operate the parallel power converters is through interleaved gating. Interleaved gating means that the switching patterns of the parallel converters are uniformly phase shifted, rather than synchronized. Interleaved gating has several advantages such as having reduced harmonic filter size, increased system efficiency, greatly enhanced control bandwidth (and thus improved dynamic performance), and potentially reduced EMI (electromagnetic interference).
Common mode current that circulates among the paralleled multiple converters or within paralleled converter systems that does not contribute to the output to the load is typically referred to as “cross current.” Both synchronous and interleaved gating control embodiments typically result in undesirable cross current with the cross current being more severe in interleaved embodiments. In ideal conditions synchronous gating does not lead to cross current, but in actual circuits using synchronous gating cross current exists due to mismatched circuit parameters. One way to reduce the cross current is by using an isolation transformer. In embodiments with isolation transformers, these isolation transformers account for almost one third of the system cost.
The existing techniques for controlling cross current without using an isolation transformer all suffer from certain inherent disadvantages. For example, using current balancers or inter-phase reactors for controlling cross current requires design of an inter-phase reactor. Such design cannot be standardized for arbitrary numbers of converters in parallel.
Another technique of controlling cross current without using isolation transformers is through use of “combined-mode” current control by treating two parallel converters as one converter, selecting the “optimum” switching vector, and adding a current balancer. The “combined-mode” approach is not suitable for more than two converters in parallel because the modulator complication level increases drastically when dealing with more than two parallel converters.
It would therefore be desirable to have an improved cross control system for interleaved or synchronous operation of multiple power converters, arranged in parallel, without using isolation transformers.